iosys_map_rd_field(9)

💅 iosys_map_rd_field(9) — like, the whole Kernel API Manual, but make it make sense

okay so bestie, welcome to iosys_map_rd_field — it's a macro from the Kernel API Manual and honestly it's giving "tiny but powerful" 💖. Its whole thing? Read a member from a struct in an iosys_map. That's it. That's the vibe check. ✨

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🌸 NAME 🌸

iosys_map_rd_field — reads a member out of a struct that lives inside the iosys_map. no cap, that's the job 🫶

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🦋 SYNOPSIS 🦋

iosys_map_rd_field (map__ , struct_offset__ , struct_type__ , field__ );

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🎀 ARGUMENTS 🎀

• map__ — the iosys_map structure itself. the main character, literally 👑
• struct_offset__ — offset from the beginning of the map, i.e. where the struct is hanging out. like a little address coordinate 🗺️
• struct_type__ — the struct that describes how the mapping is laid out. it's the blueprint, the floor plan, the whole aesthetic 🏠
• field__ — the member of the struct you actually wanna read. the one you came for 💅

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✨ DESCRIPTION ✨

So what it does is — it reads a value from iosys_map, and it figures out the layout because a C struct is describing it, starting at struct_offset__. It calculates the field offset and the size, then reads the value. Math but make it fashion. 🧮✨

⚠️ okay important, don't skip this: if the field access would incur in unaligned access, then either iosys_map_memcpy_from needs to be used, or the architecture must support it. no vibes-based guessing here, bestie. it's fr fr a real caveat 😭

For example: suppose there's a struct foo defined like below, and you need to read the value foo.field2.inner2 from the iosys_map:

struct foo {
        int field1;
        struct {
                int inner1;
                int inner2;
        } field2;
        int field3;
} __packed;

This is the expected memory layout of a buffer using iosys_map_rd_field:

┌────────────────────────────────────────────────────┐
│ Address                    Content                 │
├────────────────────────────────────────────────────┤
│ buffer + 0000              start of mmapped buffer │
│                            pointed by iosys_map    │
│ buffer + struct_offset__   start of struct foo     │
│ buffer + wwww              foo.field2.inner2       │
│ buffer + yyyy              end of struct foo``     │
│ buffer + zzzz              end of mmaped buffer    │
└────────────────────────────────────────────────────┘

The values that this macro calculates automatically — or the ones that just aren't needed — are denoted by wwww, yyyy and zzzz. cute little placeholders, they ate 🍓

And this is the code to read that value:

x = iosys_map_rd_field(map, offset, struct foo, field2.inner2);

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🔥 RETURN 🔥

The value read from the mapping. that's it, that's the whole return, and it understood the assignment 💜

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🌈 SEE ALSO 🌈

• Kernel file ./include/linux/iosys-map.h overview(9)
• struct iosys_map(9)
• IOSYS_MAP_INIT_VADDR(9)
• IOSYS_MAP_INIT_VADDR_IOMEM(9)
• IOSYS_MAP_INIT_OFFSET(9)
• iosys_map_set_vaddr(9)
• iosys_map_set_vaddr_iomem(9)
• iosys_map_is_equal(9)
• iosys_map_is_null(9)
• iosys_map_is_set(9)
• iosys_map_clear(9)
• iosys_map_memcpy_to(9)
• iosys_map_memcpy_from(9)
• iosys_map_incr(9)
• iosys_map_memset(9)
• iosys_map_rd(9)
• iosys_map_wr(9)
• iosys_map_wr_field(9)

like, the whole iosys_map friend group showed up to the party 🫶🎉

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okay that's the page!! sourced straight from ./include/linux, dated October 2026, and yeah — iosys_map_rd_field(9), that's her 💅✨